A Schottky barrier diode is a diode that produces a rectifying action utilizing a potential barrier formed at the junction between a metal and a semiconductor. Si is most widely used as the semiconductor (see Patent Literature 1, for example). As a compound semiconductor having a band gap wider than that of Si, GaAs and, recently, SiC may be used instead of Si (see Patent Literatures 2 and 3, for example).
An Si-based Schottky diode is used for a high-speed switching element, a GHz-band transmission/reception mixer, a frequency conversion element, and the like. A GaAs-based Schottky diode can implement a higher-speed switching element, and is used for a microwave converter, a microwave mixer, and the like. It is considered that SiC can be applied to the electric vehicle field, the railroad field, the power transmission field, and the like (for which a higher voltage is required) due to its wide band gap.
A Schottky barrier diode that utilizes Si is relatively inexpensive, and is widely used. However, since the band gap of Si is as narrow as 1.1 eV, it is necessary to increase the size of the element in order to improve the breakdown characteristics. The band gap of GaAs is 1.4 eV, which is wider than that of Si. However, since it is difficult to implement the epitaxial growth of GaAs on an Si substrate, it is difficult to obtain a crystal with a small number of dislocations. Since the band gap of SiC is as wide as 3.3 eV, it is considered that a high dielectric breakdown field and better performance can be achieved by utilizing SiC. However, since the substrate production process and the epitaxial growth process require a high-temperature process, SiC has a problem from the viewpoint of mass productivity and cost.
Recently, Ga2O3 has attracted attention as a material having a band gap wider than that of SiC.
An oxide semiconductor is a material that has high mobility and a wide energy gap, and the application of an oxide semiconductor to a next-generation display driver transistor, a short-wavelength sensor, a low-power-consumption circuit, and the like has been desired. Non-Patent Literature 1 reports that monoclinic β-Ga2O3 was used for a power device, and VB=0.71 MV/cm was achieved. Patent Literature 4 discloses an example in which an ohmic electrode obtained by stacking monoclinic β-Ga2O3 and Ti is applied to a light-emitting diode.
Ga2O3 may have an α, β, γ, δ, or ∈ crystal structure. Ga2O3 having a β crystal structure (monoclinic crystal structure) has the highest thermal stability, and it has been reported that the band gap of Ga2O3 having a β crystal structure is 4.8 eV to 4.9 eV. A β-Ga2O3 monocrystalline substrate can be obtained using a floating zone (FZ) method or an edge-defined film-fed growth (EFG) method. However, since it is necessary to use a molecular beam epitaxy method at present in order to implement homoepitaxial growth, there is a problem from the viewpoint of mass productivity.